This invention relates generally to transceiver architecture in a wireless mobile communication device.
Radio frequency (RF) transmitters are found in many one-way and two-way communication devices, such as mobile communication devices, (cellular telephones), personal digital assistants (PDAs) and other communication devices. A RF transmitter must transmit using whatever communication methodology is dictated by the particular communication system within which it is operating. For example, communication methodologies typically include amplitude modulation, frequency modulation, phase modulation, or a combination of these. In a typical global system for mobile communications (GSM) mobile communication system using narrowband time-division multiple access (TDMA), a Gaussian minimum shift keying (GMSK) modulation scheme is used to communicate data.
The deployment of new wireless systems presents unique challenges to mobile handset designers. In order to reap the full benefit of expanded capacity and increased data bandwidth, the new handsets must work on both the new systems as well as the old. One of these new systems has been named Enhanced Data Rates for GSM Evolution (EDGE). The EDGE standard is an extension of the Global System for Mobile Communications (GSM) standard.
The EDGE standard increases the data rate over that available with GSM by sending more bits per RF burst. More bits are sent in EDGE by using a modulation scheme based on 8-phase shift keying (8PSK), which provides an increase over GSM's Gaussian minimum shift keying (GMSK) modulation format. In the EDGE modulation scheme, the 8PSK constellation is rotated ⅜ radians every symbol period to avoid problems associated with zero crossings. In contrast to GMSK's constant amplitude envelope, the EDGE modulation scheme results in a non-constant amplitude envelope. This non-constant amplitude in the output signal presents some difficulties with regard to RF power control.
During multi-slot operation of a mobile handset transmitter using 8PSK modulation, the power of the modulated radio-frequency (RF) signal is required to ramp-up to a desired power level for a set period of time during which the handset transmits encoded data symbols. After the transmission has completed, the power of the modulated RF signal is required to return or ramp down to an off power level. The ramp-up and ramp-down must be accomplished without adversely affecting time and frequency parameters defined by the EDGE communication standard.
One conventional approach to power control generates a signal that is used to controllably adjust the gain of a variable gain amplifier located in series with a linear power amplifier. For polar loop transmitter architectures, which are already operating near saturation in 8PSK mode, power control has been accomplished through power amplifier bias controls. These conventional power controllers require integrated circuit space, increase the power budget of the mobile handset and for some conditions require a longer time than that available to meet frequency spectrum requirements.
Another approach is introduced in U.S. Patent Application Publication 2005/0249312 to Bode et al. (the '312 publication). The '312 publication describes a digital modulator that introduces dips in the envelope of the I/Q signal between adjacent time intervals or bursts. A dip-shaped waveform is multiplied with each of the I and Q waveforms to introduce the dips. A pulse-shaping filter is used with the dip-shaped waveform to obtain the desired result in the envelope of the I/Q signal. This solution requires additional memory to store the dip waveform and integrated circuit space to implement the pulse-shaping filter.
In addition to the EDGE standard, some mobile networks communicate using a code division multiple access (CDMA) standard. In a communication system using CDMA, a base station transmits control messages and voice traffic to the mobile handsets on a forward link. The mobile handsets send control messages and voice traffic to the base station on a separate reverse link. While, both the forward and reverse links require power control, the main need for power control arises in the reverse link.
The reverse link requires power control primarily to solve the “near-far” problem. In a CDMA communication system, all handsets transmit on the same frequency channel at the same time but with different codes. Therefore, one handset's signal may interfere with the others. A particular mobile's received signal quality at the base station is inversely proportional to the power of the interference from other mobile handsets. The near-far problem arises when two mobile handsets transmit at the same power but at different distances from the base station. Due to different propagation losses, the transmissions can arrive with very different received power levels. The mobile handset nearer the base station, which has a high received signal power, greatly interferes with a more distant mobile handset, which under some circumstances may not be detected.
Power control in the reverse link also deals with the rapidly changing attenuation characteristics of multipath fading channels common in urban environments. In these environments, the received power of a typical wireless channel varies dramatically with time for moving handsets and multipath characteristics.
To solve problems encountered in urban environments, an open-loop power control algorithm ensures that the received power levels of all handsets are the same at the base station. The algorithm does this by controlling the transmit power from each of the separate mobile handsets. In general, a particular handset is commanded to transmit at a higher power level when their received power is low, such as when they are far from the base station. Additionally, handsets are commanded to transmit at a lower power level when their received power is high, such as when they are near the base station.
To control power in this manner, the algorithm regularly monitors the received power of each mobile handset and commands each handset to adjust its respective transmit power to achieve predefined performance levels, such as frame error rate (FER). The base station commands each of the handsets to set their transmit power levels with predefined step sizes for making rapid changes. This open-loop control scheme is unable to respond to rapid power changes such as those that occur due to the use of DC-to-DC converters in enhanced power amplifiers.